1. Field of the Invention
This invention relates to a flashlight-radiant apparatus suitable for use, for example, in the heat treatment of semiconductor wafers.
2. Description of the Prior Art
An ion implantation process, for example, a process in which phosphorous ions at high energy are bombarded and introduced in a semiconductor wafer made of silicon, is practiced in order to promote the crystallization of the wafer or impart useful characteristics to the wafer. Such an ion implantation however develops crystal defects in the wafer. It is thus necessary to subject the wafer to a heat treatment to anneal same so that crystal defects developed can be healed. Furthermore, it is also required to heat-treat a wafer for forming a single-crystalline silicon layer by use of a method of gas-phase epitaxial growth. Heretofore, such heat treatments of wafers have usually been carried out by heating wafers in a resistive furnace while causing an inert gas such as nitrogen gas to flow therethrough. However, the heat treatment in a resistive furnace is accompanied by such drawbacks that it requires a long heating time period, it is difficult to heat a wafer of a large surface area uniformly, it develops "warping" in a wafer, and/or it tends to cause contamination of a surface layer of a wafer. As excellent heat treatment apparatus free of such drawbacks, flashlight-radiant apparatus making use of flash discharge lamps have recently attracted engineers' attention.
Such a flashlight-radiant apparatus however requires high output flash discharge lamps and is very disadvantageous if one tries to carry out a heat treatment of a cold wafer of room temperature by heating it to a desired high temperature only with the energy applied by the flash discharge lamps to the wafer. Therefore, it is indispensable, from the practical viewpoint, to heat each wafer additionally upon heating same by a flash of light from flash discharge lamps. In order to conduct such additional heating, it is preferred to provide subsidiary heating means as a unitary member with a wafer-supporting table, to place the wafer-supporting table below flash discharge lamps and then to turn on the flash discharge lamps at one time to have the lamps to radiate a flash of light while additionally heating each wafer. In such a structure as mentioned above, the flash discharge lamps are however heated to high temperatures by heat which is generated continuously from the subsidiary heating means. Thus, use of such subsidiary heating means is accompanied by another drawback that flash discharge lamps are heated to high temperatures owing to the heat given continuously off from the subsidiary heating means, thereby deteriorating the flash discharge lamps at an early stage and hence shortening their service life considerably.
Furthermore, a flash discharge lamp is operated by applying a high voltage thereto. It is thus an important issue to assure the safety of the apparatus for high voltages. However, it is still dangerous from the standpoint of safety to handle a wafer on a supporting table placed right below flash discharge lamps.